This is the 1441st most frequent Portuguese word.
qualquer
"Any" or "whatever."
Here, 'qualquer' is used as an adjective meaning 'any', modifying 'livro' ('book') to imply that any book can be chosen.
Você pode escolher qualquer livro da prateleira.
You can choose any book from the shelf.
In this sentence, 'qualquer coisa' means 'anything', showing an indefinite sense about what was done.
Ela fez qualquer coisa para ajudar os amigos.
She did anything to help her friends.
'Qualquer pessoa' here means 'anyone', indicating an unspecified or general individual.